Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller-cone earth-boring rotary drill bits include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which the cone is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutters, which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high pressure and high temperature, conventionally in the presence of a catalyst (such as cobalt, iron, nickel, or alloys and mixtures thereof), to form a layer of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high pressure/high temperature (or “HPHT”) processes, which may take place at a pressure of about 5.5 to about 7.2 GPa and at a temperature of about 1200° C. to about 2200° C. Catalyst material is conventionally mixed with the diamond grains to reduce the amount of oxidation of diamond by oxygen and carbon dioxide during an HPHT process and to promote diamond-to-diamond bonding.
The cutting element substrate may include a cermet material (i.e., a ceramic-metal composite material) such as cobalt-cemented tungsten carbide. In such instances, the cobalt (or other catalyst material) in the cutting element substrate may be drawn into the diamond grains or crystals during sintering and serve as a catalyst material for forming a diamond table from the diamond grains or crystals. In other methods, powdered catalyst material may be mixed with the diamond grains or crystals prior to sintering the grains or crystals together in an HPHT process. In additional methods, thin bodies (e.g., foils, disks) of the catalyst elements are stacked adjacent to the diamond grains for infusion into the spaces between the diamond grains during HPHT processing. Cutting elements formed by using such thin bodies often results in a non-homogenized (e.g., non-uniform) distribution of the catalyst elements throughout the resulting diamond table.
Upon formation of a diamond table using an HPHT process, catalyst material may remain in interstitial spaces between the grains or crystals of diamond in the resulting polycrystalline diamond table. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use, due to friction at the contact point between the cutting element and the formation.
Conventional PDC formation relies on the catalyst alloy, which sweeps through the compacted diamond feed during HPHT synthesis. Traditional catalyst alloys are cobalt-based with varying amounts of nickel, tungsten, and chromium to facilitate diamond intergrowth between the compacted diamond material. However, in addition to facilitating the formation of diamond-to-diamond bonds during HPHT sintering, these alloys also facilitate the formation of graphite from diamond during drilling. Formation of graphite can rupture diamond necking regions (i.e., grain boundaries) due to an approximate 57% volumetric expansion during the transformation. This phase transformation is known as “back-conversion” or “graphitization,” and typically occurs at temperatures approaching 600° C. to 1,000° C., which temperatures may be experienced at the portions of the PDC contacting a subterranean formation during drilling applications. This mechanism, coupled with mismatch of the coefficients of thermal expansion of the metallic phase and diamond, is believed to account for a significant part of the failure of conventional PDC cutters to meet general performance criteria known as “thermal stability.”
To reduce problems associated with different rates of thermal expansion and with back-conversion in polycrystalline diamond cutting elements, so-called “thermally stable” polycrystalline diamond (TSP) cutting elements have been developed. A TSP cutting element may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the diamond grains in the diamond table using, for example, an acid. Substantially all of the catalyst material may be removed from the entire diamond table, or from only a portion of the diamond table, for example the cutting face, and in some instances from a portion of the sidewall of the diamond table. TSP cutting elements in which substantially all catalyst material has been leached from the diamond table have been reported to be thermally stable up to temperatures of about 1,200° C.